This invention is concerned generally with oscillators, and more particularly with resonator oscillators that generate a signal within a selected band of frequencies having no unwanted or spurious outputs above or below the selected band frequencies.
A typical resonator oscillator will oscillate not only at the frequency for which it was designed, but also at certain harmonics and sub-harmonics of that frequency, plus other spurious frequencies. To separate the desired frequency from the others, various mode suppression techniques have been employed in the past. These techniques include: resonant traps to suppress each unwanted frequency; low pass traps for frequencies below the desired frequency; and post oscillator filters.
Each of these approaches is a limited, incomplete, solution to the problem. The resonant trap approach requires that each of the unwanted frequency modes be identified and a separate filter be included in the circuit to suppress each of those modes. This results in the addition of a large number of additional parts which quickly increases the resistive loss in the circuit.
The low pass trap approach, while requiring fewer additional parts than the resonant trap approach, only removes the undesired modes below the desired mode. There is no suppression of the higher frequency modes. A Pierce oscillator of the type described on page 70 and shown in FIGS. 7-2 on page 59 of Crystal Oscillator Design And Temperature Compensation by Marvin E. Frerking, copyright 1978 is illustrative of this type of mode suppression.
Post oscillator filters, while often providing a satisfactory solution to the problem, probably more times than not do not present a satisfactory solution. The downfall of this solution comes when the desired frequency mode is weak and is dominated by one or more stronger, unwanted modes. Murphy's law tells us that it is more likely that the mode we are interested in will be the weak mode.
To overcome the shortcomings of each of the abovementioned techniques, the circuit of the oscillator feedback path should be such that the phase of the feedback signal is 180.degree. only for a narrow band of frequencies which includes the desired frequency. Through the use of such an approach, any number of unwanted oscillation modes both above and below the desired oscillation frequency could be suppressed successfully. The instant invention presents such an approach.
In accordance with the preferred embodiment, the present invention includes an inverting amplifier and a feedback path between the input and output terminals of the amplifier. This feedback path includes a series reactive element in the form of a crystal resonator and a pair of shunt reactive elements one end of each being connected to the two ends of the crystal resonator and the second ends of the shunt elements connected together to form a common line.
There are two basic embodiments of the present invention. They are a two-arm bandpass mode suppression configuration and a one-arm bandpass mode suppression configuration. In the two-arm configuration one of the shunt reactive elements includes a parallel resonant tank and the second of the shunt reactive elements includes a series resonant tank. Each of these tanks have a resonant frequency at which the sign of the reactance of the tank switches from positive to negative. By the proper choice of the values of the capacitors and inductors in each of these tanks these two resonant frequencies can be made to define a band of frequencies substantially between which the feedback path provides a phase shift of 180.degree..
In the one-arm bandpass mode suppression configuration one of the shunt elements includes a non-resonant reactive circuit, while the other shunt element includes a resonant tank with both a series and a parallel resonant path. In this configuration the series resonant frequency and the parallel resonant frequency of that shunt element define the band of frequency between which oscillation is possible.
Each of these configurations lend themselves equally to both the Colpitts or Pierce and the Hartley oscillator configurations. In each of these basic oscillator configurations, a feedback path phase shift of 180.degree. is only possible when the sign of the series reactance element is different from the sign of both of the shunt reactance elements. To achieve this within the desired band of frequencies, the sign of the effective reactance of the shunt reactive elements must match in that range. This can be achieved again by the proper selection of the values of the capacitors and inductors that are used in the shunt reactive element tank circuits.